Acquisition of KCG - Narrative (Details) - USD ($)				1 Months Ended	12 Months Ended
	Aug. 10, 2017	Jul. 21, 2017	Jul. 20, 2017	May 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Aggregate purchase price						$ 735,974,000
Employee compensation expense						6,400,000
Annual incentive compensation payments						35,300,000
Goodwill					$ 836,583,000	844,883,000
Deferred tax assets					200,359,000	125,760,000
Market Making
Business Acquisition [Line Items]
Goodwill			$ 96,200,000		755,292,000	755,292,000
Execution Services
Business Acquisition [Line Items]
Goodwill			$ 32,100,000		$ 81,291,000	$ 89,591,000
Senior Secured Notes Due 2020
Business Acquisition [Line Items]
Interest rate		6.875%
Redemption price as a percentage of the principal amount		103.438%
Face amount		$ 465,000,000.0
Temasek Investment
Business Acquisition [Line Items]
Other offering costs	$ 7,800,000
Class A
Business Acquisition [Line Items]
Common stock, par value (in dollars per share)					$ 0.00001	$ 0.00001
Number of shares of stock issued				10,518,750
Employee compensation expense					$ 11,200,000	$ 11,000,000
Class A | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued	338,124
Aggregate purchase price	$ 5,200,000
KCG
Business Acquisition [Line Items]
Common stock, par value (in dollars per share)			$ 0.01
Right to receive cash (in dollars per share)			$ 20.00
Cash purchase price			$ 1,400,000,000
Fair value (in dollars per share)			$ 20.00
Goodwill			$ 128,286,000
Deferred tax assets			$ 23,900,000
KCG | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued			39,725,979
Aggregate purchase price			$ 613,500,000
KCG | Class A | Aranda
Business Acquisition [Line Items]
Number of shares of stock issued			6,346,155
Aggregate purchase price			$ 99,000,000
KCG | Class A | Temasek Investment
Business Acquisition [Line Items]
Number of shares of stock issued	1,666,666
Aggregate purchase price	$ 26,000,000
KCG | Class A | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued	338,124
Aggregate purchase price	$ 5,200,000